docs: def error_string(self):
          """Returns a well formed error message
  
          e.g at least 5 times but was called 4 times
  
          :rtype string
          """
  
          if self.has_correct_call_count():
              return ''
  
          return '{} instead of {} {} '.format(
              self._restriction_string(),
              self.count,
              pluralize('time', self.count)
          )